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charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 18B44C0014 X-Stat-Signature: xoc1dq7na9kk33mz39u9rtfaqwd5zy1y X-Rspam-User: X-HE-Tag: 1722827883-302982 X-HE-Meta: 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 ox73+vK0 aHJLOqYSrBYpE5eUeDhW2C6QG6LDJDrSJvhoN+Y/DEAPgS+UK/FXWRiNIHUXD3LrD1fMcprdJX/X+7l+aoDkKeLO862tKAfH7cFEqPOx/LlKuiHJvc1kW4cZiw796Beq5MaHOAFnvVNvcQ1ATcNvmlBkTC1Nh0qS5d3HZx8rjCxB0rByjCYX/ZrLJssKLKPmYGcKVeohoOQm+RC98rY64HLEzVIv5M1Vb9Yxdq5GqS0f/RMmfEXmrRzDo7DwrYrMHr/1a7EpM5WDgbQLtXQEIuBfarIMO88Fdwv43q/mcMNCW9weonP4RSB9yXo6PR3FU8calNXD5iTZWXXbNJ9aJEwBmiQErXdj4ro41Pe8TsJq5ZYDSXxQTxt8uCKDOC8TDbYMIbj4mwBPnQjU8V5wioz6R6gCiQ+eYS/Eo X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 5, 2024 at 11:05=E2=80=AFAM Huang, Ying = wrote: > > Yafang Shao writes: > > > On Mon, Aug 5, 2024 at 9:41=E2=80=AFAM Huang, Ying wrote: > >> > >> Yafang Shao writes: > >> > >> [snip] > >> > >> > > >> > Why introduce a systl knob? > >> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D > >> > > >> > From the above data, it's clear that different CPU types have varyin= g > >> > allocation latencies concerning zone->lock contention. Typically, pe= ople > >> > don't release individual kernel packages for each type of x86_64 CPU= . > >> > > >> > Furthermore, for latency-insensitive applications, we can keep the d= efault > >> > setting for better throughput. > >> > >> Do you have any data to prove that the default setting is better for > >> throughput? If so, that will be a strong support for your patch. > > > > No, I don't. The primary reason we can't change the default value from > > 5 to 0 across our fleet of servers is that you initially set it to 5. > > The sysadmins believe you had a strong reason for setting it to 5 by > > default; otherwise, it would be considered careless for the upstream > > kernel. I also believe you must have had a solid justification for > > setting the default value to 5; otherwise, why would you have > > submitted your patches? > > In commit 52166607ecc9 ("mm: restrict the pcp batch scale factor to > avoid too long latency"), I tried my best to run test on the machines > available with a micro-benchmark (will-it-scale/page_fault1) which > exercises kernel page allocator heavily. From the data in commit, > larger CONFIG_PCP_BATCH_SCALE_MAX helps throughput a little, but not > much. The 99% alloc/free latency can be kept within about 100us with > CONFIG_PCP_BATCH_SCALE_MAX =3D=3D 5. So, we chose 5 as default value. > > But, we can always improve the default value with more data, on more > types of machines and with more types of benchmarks, etc. > > Your data suggest smaller default value because you have data to show > that larger default value has the latency spike issue (as large as tens > ms) for some practical workloads. Which weren't tested previously. In > contrast, we don't have strong data to show the throughput advantages of > larger CONFIG_PCP_BATCH_SCALE_MAX value. > > So, I suggest to use a smaller default value for > CONFIG_PCP_BATCH_SCALE_MAX. But, we may need more test to check the > data for 1, 2, 3, and 4, in addtion to 0 and 5 to determine the best > choice. Which smaller default value would be better? How can we ensure that other workloads, which we haven't tested, will work well with this new default value? If you have a better default value in mind, would you consider sending a patch for it? I would be happy to test it with my test case. -- Regards Yafang